The lower limit of the vehicle electrical system voltage of a motor vehicle is primarily a function of the charge condition of the battery and the rate of the charging current of the dynamo for a vehicle electrical system with a nominal or standard voltage of 24 volts. It has been found that, after a motor vehicle is started with a significantly run-down battery, and only a small battery charging current rate, the voltage will readily drop to 20 volts or less only after a short period of running time. It will be appreciated that voltage drops through lines and over the contact points will also reduce the available amount of voltage. However, these voltage drops are generally less than 1 volt.
When vehicles have significantly rundown batteries and the current demands of the loads are relatively high, e.g., heaters, fans, headlights or an anti-skid system are turned ON immediately after the motor is started; and if the current requirement is not thereby met by the dynamo, an unpredictable undervoltage can occur. The same thing occurs, however, with a consumer requiring only a small current, if the motor is turned OFF or "killed" immediately after being started, and the battery cannot satisfy the small current requirements because of its rundown condition.
For these reasons, a lower limit supply voltage value of 20 volts is specified for the electronic system of an anti-skid system for a 24-volt vehicle electrical system. On the solenoid valves for control of the braking pressure, which are activated by the electronic system actuators, an 18-volt limit is specified as the lower value; so that, in spite of the voltage drop at the actuators, operation of the system is guaranteed even with a weak vehicle electrical system.
If the above-mentioned lower voltage limit of 20 volts is not achieved in the vehicle electrical system of the motor vehicle, it is not possible to guarantee correct operation of an anti-skid system. A previously known anti-skid brake control system of this type is shown and disclosed in published German patent application DE-A No. 22 53 867 which substantially corresponds to U.S. Pat. No. 3,874,743. A disadvantage of this arrangement, however, is that the anti-skid action will cease to operate. If the disengagement takes place during a controlled braking, all the wheels lock up suddenly since, in such a case, the normal braking action takes over.
Another disadvantage of the previously known systems, is that in high-ohmage vehicle electrical systems, the voltage can again increase beyond the threshold voltage after the anti-skid system is turned OFF. If, thereby, the anti-locking system is immediately turned back ON, cycling phenomenon can occur, i.e., the system is continuously turned ON and OFF. In order to prevent this from occurring, it is necessary that the reactivation should only be permitted when the voltage exceeds the threshold value, i.e., by providing a hysteresis.